The present invention relates generally to an apparatus and method for determining bond integrity (adhesion status) of joined materials at a micro-level.
There has been a long-felt need for non-destructive inspection methods that can be used to determine the integrity of a bond between two items. This need has been felt particularly in the case of bonds formed at the micro-level. The micro-level can be defined as involving at least one material with a dimension on the order of 0.005 inch or smaller. Such inspection methods might be applied to testing ball and wedge bonds, thin coatings, circuit traces, ribbon bonds, solder balls, surface mount components, PIN grid arrays, and MIMMs commonly used in microelectronics interconnects. The materials joined in these applications include, but are not limited to, silicon, silicon carbide, aluminum, gold, gallium arsenide and the like.
For example, ball bonds used in connecting silicon wafers to external circuits through very fine wires are typically tested, if at all, according to military specifications which require a pull test of each bond. This test is performed using a machine which sequentially hooks each wire and applies a predetermined pulling force to determine whether the associated bond will hold. This technique has significant limitations. In particular, the inventor has discovered that if this test is performed repeatedly on the wires of the same device, an increasing number of wires typically pull loose with each succeeding test. This result implies that the test does not truly qualify as non-destructive. That is, each application of a pulling force to a wire weakens its bond and repeated testing will actually break the bonds. It is possible that a bond might pass a single pull test of this type, but that the test would leave the bond precariously connected and destined for failure in the field when subjected to ambient vibration, shock, or temperature variations.
In the case of bonds having larger dimensions, such as pipe seams, welds used in automotive and marine manufacturing, etc. various x-ray and acoustic techniques have been applied to analyze the condition of an interface between two items. Laser ultrasound techniques have also been proposed. For example, U.S. Pat. No. 4,659,224 and U.S. Pat. No. 4,966,459 to Monchalin, U.S. Pat. No. 5,081,491, to Monchalin et al., and U.S. Pat. No. 5,137,361 to Heon et al. disclose the results of early research in this field. U.S. Pat. No. 5,103,676 to Garcia et al. shows a further method of laser ultrasonic process monitoring.
Laser techniques have also been considered for use with smaller bonds such as those found in semiconductor circuits. U.S. Pat. No. 5,201,841 to Lebeau et al. proposes a thermal gradient technique, and Japanese Patent Publication 62-7198 Jan. 14, 1987 by Hitachi Research Corp. appears to propose a laser technique.
The inventor""s prior U.S. Pat. Nos. 5,420,689, 5,424,838, and 5,302,836 disclose lighting methods and apparatus useful in small-scale laser ultrasonic measurement. U.S. Pat. No. 5,535,006 to Siu et al. builds on the inventor""s earlier work and discloses a method of evaluating integrity of adherence of a conductor bond to a substrate.
However, as far as the inventor is aware, none of these prior systems has provided an effective alternative to pull testing of wire bonds, or an effective method for analyzing thin film coating integrity at the micro-level. The results produced by the systems disclosed in the inventor""s own prior patents, while encouraging, were not consistent enough for industrial application.
Thus, there is a need in industry for improved methods and systems of this type that will provide repeatable, accurate, and truly non-destructive testing capability.
Therefore, it is a general object of the invention to provide an improved system and method for determining the bond integrity (adhesion) status of adjoining materials using laser ultrasonic techniques.
It is another general object of the invention to provide an improved system and method for determining the bond integrity (adhesion) status of adjoining materials using a pulse laser which applies heat onto the surface of interest, resulting in generation of a thermoelastic propagation (surface, bulk, air waves or combination thereof) in all directions from the pulse point, which can be detected using a stabilized continuous wave laser using interferometric techniques.
Another broad object of the invention is to provide an improved system and method that is particularly adapted to nondestructively test and evaluate the thickness and/or uniformity and adhesion characteristics of a thin coating of material applied to a substrate.
Another general object of the invention is to provide an improved system and method that is particularly adapted to nondestructively determine the bond integrity of joining materials at the micro-level, such as microelectronic interconnects, ball bonds, wedge bonds, circuit traces, surface mount components and MIMMs.
It is also an important object of the invention to provide an improved system and method for analyzing thermoelastic propagation signatures, single or in combination, to interpret bond integrity (adhesion) of adjoining materials, and determine whether they are fully bonded, partially bonded and touching yet non-bonded.
Another useful object of the invention is to provide a fully automated bond integrity determination system with particular applications in inspection and testing in microelectronics manufacturing processes.
An additional object of the invention is to provide improved operational timing and signature gathering methods and apparatus for use in a laser ultrasonic measuring system.
It is also an object of the invention to provide a laser ultrasonic measuring system with a vastly improved signal to noise ratio for detected wave propagation signatures.
A further object of the invention is to provide a method for correlating surface wave propagation to bond integrity in the context of a bond testing system.
These objects and others are achieved, in a preferred embodiment of the present invention, by providing a pulse laser and a continuous laser detector forming a cause and effect sensing device. The pulse laser sends a single or multiple pulse(s) of controlled magnitude and bombards the object of interest causing a thermoelastic excitation response. This excitation in turn induces an ultrasonic propagation along or through the surface material. By detecting, capturing and interpreting these thermoelastic propagation signatures, the attachment condition of the joining materials is determined. The technique is a significant improvement over traditional mechanical pull, shear or contact type techniques. The object need not be contacted by mechanical means, the excitation is much gentler than that required in a contact test, and the speed of the test is much faster than other automated manufacturing process, making it suitable for real-time process control purposes.